Because carbon fibers prepared from pitch-type materials involve lower production costs than carbon fibers produced from organic synthetic fibers such as polyacrylonitrile as a precursor and because articles of high elastic modulus can be easily produced from the pitch-type carbon fibers, it is expected that pitch-type carbon fibers can be less expensive then high-performance materials. However, commercially available pitch-type carbon fibers have a tensile strength of not more than about 200 kg/mm.sup.2 and poor quality stability, hence not satisfactory.
Generally the cohesion stage of molecules in section of pitch-type carbon fibers (hereinafter referred to as high-order sectional structure) widely varies with spinning conditions. More specifically, this structure is basically classified into the type in which molecules form crystals along the concentric circles of fiber (so-called onion type), the type in which molecules form crystals radially of center of fiber (radial type), and the type in which molecules are randomly distributed without orientation (random type). Yet there actually exist fibers having a structure of mixtures of these types. Further, flaws such as longitudinal fractures, cracks, voids and the like may exist in part or whole of the fibers. Accordingly the high-order sectional structures of pitch-type carbon fibers including such flaws are complicated and abundantly various. The presence of various flaws and high-order sectional structures is one of the main causes of lowering the quality stability of pitch-type carbon fibers.
While depending on the properties of pitch for use in spinning, the occurrence of the above-mentioned flaws and the formation of high-order sectional structures are most greatly affected and varied by the spinning conditions. Accordingly, in order to improve the quality and stability of carbon fibers, it is necessary to establish spinning techniques capable of invariable producing carbon fibers substantially free of flaws and having a specific high-order sectional structure even if these are some irregularities in properties of pitch for spinning. In other words, techniques are required for stably forming the onion type and/or random type of high-order sectional structures which are effective in exhibiting a high tensile strength without creating flaws such as longitudinal fractures, cracks, voids or the like.